Macrocyclic Peptide Modulators of Protein Function Progress toward an understanding of protein-mediated interactions involved in cell physiology and disease heavily relies on the availability of chemical agents capable of targeting and modulating protein function with high potency and selectivity. While small-molecule agents have provided a major source of chemical probes and therapeutics, an overwhelming fraction of human proteins and protein-mediated interactions remains impervious to modulation using this class of molecules, posing a fundamental barrier to efforts directed at elucidating their role in physiopathological processes and assessing their therapeutic potential. Our group has recently introduced an efficient and highly versatile methodology for generating small- size, genetically encoded macrocyclic peptides in living cells and demonstrated the effectiveness of these 'natural product- like' compounds to disrupt a challenging protein-protein interaction with high potency and specificity. In this project, this versatile methodology for macrocyclic peptide synthesis will be integrated with phage display and cell-based selection systems in order to implement powerful, high-throughput platforms for the rapid discovery of potent and selective macrocyclic peptide modulators of proteins and protein-protein interactions. Successful completion of this research is expected to make available new, efficient, and readily accessible technologies useful for developing macrocyclic peptide agents capable of modulating protein function with high potency and selectivity across a wide range of target proteins and cellular processes. As such, these technologies are bound to streamline the development of chemical probes for interrogating cellular pathways and validating therapeutic targets, the generation of selective reagents for protein detection and labeling, and the identification of potential lead structures for drug development, thereby accelerating efforts in basic biomedical research, chemical biology, and drug discovery.